1. Field of the Invention
The present invention relates to a rack housing manufacturing method and a rack housing manufactured by the manufacturing method.
2. Description of Related Art
In steering devices for automobiles etc., a rack housing is used to fix a rack shaft to a vehicle body frame while supporting the rack shaft so as to be linearly reciprocally movable. The rack housing is formed to be long in the axial direction of the rack shaft, and receives a steering reaction force transferred from wheels during steering. Therefore, in general, the rack housing as a whole is formed integrally from die-cast aluminum, which is excellent in strength and rigidity, or the like.
With a demand for energy saving in recent years, however, there is also a desire for weight reduction of automobile components. Therefore, it is also considered to reduce the weight of the rack housing, which occupies a large weight ratio in the total weight of the steering device, in addition to securing necessary strength and rigidity. For example, the rack housing is divided into a tubular portion for insertion of the rack shaft, and coupling portions provided at both ends of the tubular portion to be coupled to different members. It is considered to reduce the weight of the rack housing by forming the tubular portion, among such components, from fiber-reinforced composite materials such as Carbon Fiber Reinforced Plastics (CFRP) obtained by impregnating carbon fibers with a thermoplastic resin.
The coupling portions, which are provided with portions for insertion of a pinion shaft, insertion of a rack guide (support yoke), and attachment to a vehicle body, may be formed from die-cast aluminum or the like as in the related art, and coupled to the tubular portion through screwing, heli-sert insertion, metal screw integral forming, or the like. In the invention described in Japanese Patent Application Publication No. 2013-208927 (JP 2013-208927 A), metal rings that constitute at least openings of the portions for insertion of a pinion shaft, insertion of a rack guide, and attachment to a vehicle body are set to a mold together with a tubular portion formed from fiber-reinforced composite materials, and a thermoplastic resin is injected for injection molding to form a rack housing. This further reduces the weight of the rack housing.
However, it takes a long time to form a tubular portion using fiber-reinforced composite materials containing a thermoplastic resin such as the CFRP. That is, it takes approximately five hours or more since the start of heating until the completion of the tubular portion with the thermoplastic resin completely cured, including a fluidization time in which sheet-shaped prepregs of the fiber-reinforced composite materials are heated and softened to be formed into a tubular shape, and a temperature rising time in which the prepregs which have been shaped into a tubular shape are further heated to the curing temperature of the thermoplastic resin.
The period of five hours is significantly long considering that the cycle time desired for the manufacture of automobile components is about one minute. In addition, a large amount of energy is consumed to keep heating the components during the period. Such factors may lead to a cost increase.